Method for the preparation of highly densified superconductor massive bodies of MgB2, relevant solid end-products and their use

ABSTRACT

A method is described for the preparation of superconductor massive bodies of MgB 2 , having a density close to the theorical value, which comprises the following passages: mechanical activation of crystalline boron with the formation of activated powders; formation of a porous preform of said powders; assembly of the porous boron preform and massive precursors of metallic magnesium in a container and sealing thereof in an atmosphere of inert gas or with a low oxygen content; thermal treatment of the boron and magnesium as assembled above, at a temperature higher than 700° C. for a time greater than 30 minutes, with the consequent percolation of the magnesium, in liquid phase, through the activated crystalline boron powders.

CROSS-REFERENCE TO RELATED APPLICATIONS

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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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REFERENCE TO A MICROFICHE APPENDIX

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BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to a method for the preparation of highlydensified superconductor massive bodies of Mgb₂, the revant solid endproducts and their use.

2) Description of Related Art

It has recently been discovered that magnesium boride has superconductorproperties up to 39 K and can therefore be applied in closed circuitcryogenic systems (cryo-refrigerators), which are less costly than thosebased on the use of liquid helium (Nagamatsu et al., Nature, 410, 63;2001).

Like all borides, magnesium boride, a compound which has been known forabout half a century, is characterized by extreme hardness when it ishighly densified.

The densification of magnesium boride however into end-products,reaching values close to 100% of its theoretical density (2.63 g/cm³),effected by the compacting of the powders of the compound itself,normally requires the use of high pressures. Pressures in the order ofseveral GPa are generally used.

Alternative synthesis methods of the compound MgB₂ starting fromstoichiometric, or non-stoichiometric, mixtures of boron and magnesium,both in powder form and in the form of massive bodies, are also known inliterature. In the latter case, however, the use of high pressures isindispensable for obtaining highly densified end-products.

An example is described by Canfield et al., whereby, MgB₂ fibres areobtained, starting from boron fibres reacted with liquid Mg or in vapourphase, (Phys. Rev. Lett. 86, 2423 (2001)), having an estimated densityof about 80% of the theoretical value.

It is consequently only possible to obtain an end-product of magnesiumboride densified up to values close to the theoretical value, andtherefore characterized by improved superconductivity and mechanicalproperties, with the methods of the known art, by the use of highpressures at a high temperature.

The use of high pressures at a high temperature however limits thedimensions of the end-products obtained and necessitates the use ofequipment which is not suitable for a mass production.

BRIEF SUMMARY OF THE INVENTION

An objective of the present invention is therefore to obtainsuperconductor massive bodies of MgB₂ with a density close to thetheoretical value with a method which overcomes the drawbacks present inthe known art.

An object of the present invention relates to a method for thepreparation of superconductor massive bodies of MgB₂, having a densityclose to the theoretical value, which comprises the following passages:

-   a) mechanical activation of crystalline boron with the formation of    activated powders;-   b) formation of a porous preform of activated powders of crystalline    boron;-   c) assembly of the porous boron preform and massive precursors of    metallic magnesium in a container and sealing thereof in an    atmosphere of inert gas or with a low oxygen content;-   d) thermal treatment of the boron and magnesium as assembled above,    at a temperature higher than 700° C. for a time greater than 30    minutes, with the consequent percolation of the magnesium, in liquid    phase, through the activated crystalline boron powders.

A further object of the present invention relates to a superconductormassive body or solid end-product of MgB₂, having a density close to thetheoretical value, obtained by means of the method of the presentinvention.

Another object of the present invention also relates to a method whichcomprises in step c) the use of magnesium mixed with one or morelower-melting metals, such as Ga, Sn, In, Zn, or an Mg-based alloy withsaid metals.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 compares X-ray diffraction diagrams of activated andnon-activated boron powders;

FIG. 2 is a diagram of the container and protective sheaths used inExample 2;

FIG. 3 is an X-ray diffraction pattern of the product of Example 2;

FIG. 4 is a graph showing the AC susceptibility of the product ofExample 2; and

FIG. 5 is a graph showing the AC susceptibility of the product ofExample 4.

DETAILED DESCRIPTION OF THE INVENTION

The present invention also relates to the use of the massive bodies ofMgB₂ obtainable with the method according to the present invention forsuperconductors to be used as electric current cut-ins, variableinduction elements in current limitation systems, permanent magnets tobe used in levitation systems, in medical magnetic resonance systems, inelementary particle accelerators and detectors, in energy accumulationsystems, in linear or non-linear motors, in power generators.

The fundamental advantage of the method according to the presentinvention lies in the fact that it allows the production, in a simpleand economic way, of solid superconductor end-products of MgB₂,densified up to values close to the theoretical value, with improvedcharacteristics with respect to the products obtainable with the knownmethods in the state of the art. From an applicative point of view, theuse of MgB₂, densified up to values close to the theoretical value, thusobtained, allows the current which can be conveyed to the superconductorproducts to be increased and also improves the mechanical properties ofsaid end-products.

A further advantage also consists in the fact that highly densifiedtargets of MgB₂ allow deposition technologies such as laser ablation orradio-frequency sputtering, to be applied with greater success, toobtain superconductor material deposited on substrates of variousorigins, in the form of thin films.

In particular, the method for the production of superconductor massivebodies of MgB₂, having a density close to the theoretical value, i.e. adensity higher than or equal to 2.25 g/cm³, consists in reacting theboron and magnesium elements in a sealed container in an atmosphere ofinert gas or with a low oxygen content (lower than 20% atomic), at ahigh temperature, wherein at least the boron is present in the form ofpowders, defined as active, with a suitable particle-size and having atleast two crystalline phases similar to unit cells of the rhombohedraltype.

The mechanical activation step a) crystalline boron flakes havingdimensions of a few millimeters and a purity higher than or equal to99.4%, preferably consists in a repeated crushing by high loadcompression, under “almost static” conditions, as for example can beeffected in a hydraulic press. This activation not only minimizes thepowder fraction with a finer particle-size (for example lower than 20micrometres) which is the typical grinding product of a rotating ballmill, but also allows a powder to be obtained, which maintains thecharacteristics of the crystallinity type present in the startingflakes, thus making the powders more permeable to liquid magnesium.

In particular, the activated crystalline boron powders are selected soas to have an average volumetric particle diameter ranging from 30 to 70microns and are practically without oxygen contamination. Step b)comprises the formation of a porous preform of activated crystallineboron powders. The porous preform of activated crystalline boron powdershas a shape similar to that of the end-product and must have an apparentdensity higher than 50% of the theoretical density of the crystallineboron (2.35 g/cm³).

The preform of activated crystalline boron powders may alternativelycontain up to 20% atomic of magnesium. In this case, the preformprevalently consists of activated crystalline boron powder and magnesiumpowder practically without oxygen contamination and a particle-sizelower than that of the boron. The preform can also consist of activatedcrystalline boron powders, surface-covered by metallic Mg and welded toeach other by thermal treatment in an inert atmosphere, so as tomaintain the porosity of the preform and at the same time providemechanical consistency for its handling.

Preforms containing magnesium must also satisfy the requisite ofapparent density defined above.

The following step c) comprises the assembly of the components whichwill undergo thermal treatment and transformation to the end-product instep d). The container in which these components are assembled, is alsoimportant.

Step c) comprises the insertion in a suitable container of a combinationof two components: the first component is the preform produced with theabove-mentioned activated crystalline boron powder, having a purity atleast higher than or equal to 99.4%, which has a shape similar to thatof the end-product and an apparent density higher than 50% of thetheoretical density of rhombohedral crystalline boron (2.35 g/cm³),preferably ranging from 51% to 56%. The second component consists of oneor more massive bodies of metallic Mg having a purity higher than 99%which in step d), after melting, percolates through the activatedcrystalline boron powder.

The magnesium in liquid phase preferably derives from the melting ofmassive precursors of metallic Mg. It is also practically free fromoxygen contamination.

The proportions between Mg and B largely depend on the technologyselected for carrying out the reaction. In any case they are far fromthe stoichiometric values of the MgB₂ compound. In particular, there isan excess of Mg which is such that the atomic ratio Mg/B is greater than0.5, preferably said ratio is greater than or equal to 0.55.

When mixtures of Mg with other metals are used, the atomic ratio(metals+Mg)/B should be greater than 0.55, with Mg/B contemporaneouslygreater than 0.5.

Atomic ratio values Mg/B, or (metals+Mg)/B, lower than the limitsdefined above, cause a reaction which produces a partial densificationof the product, reducing or completely cancelling the superconductivitycharacteristics relating to the conveying of the electric current.

The container in which step c) is effected, consists of a material whichcannot be attacked by boron and magnesium at temperatures up to 1000°C., such as Nb, Ta, MgO, BN, etc. or any material resistant to hightemperatures, internally lined by a sheath of one of the above materialsin order to prevent contamination of the boron preform and massivebodies of Mg due to the elements forming the container. An example ofsaid container is provided in FIG. 2.

The container must be kept sealed and structurally unaltered during thewhole treatment time of step d). An atmosphere of inert gas or,alternatively, an atmosphere with a low oxygen content (less than 20%atomic) must be present inside the container, at a pressure which issuch as to ensure the presence of magnesium in liquid phase during thewhole treatment of passage d). The sealing and mechanical integrity ofthe container can be effected by means of welding and/or by fixing in asuitable machine capable of counter-balancing the internal pressurewhich is generated during the reaction and capable of preventingcontamination with atmospheric oxygen.

Step d) of the method comprises thermal treatment at a temperaturehigher than 700° C. for a time of at least 30 minutes, in the presenceof an atmosphere of inert gas, to allow the consequent percolation ofthe magnesium, prevalently in liquid phase, through the preform ofactivated crystalline boron powder. Step d) is preferably carried out attemperatures ranging from 800° C. to 1000° C. for 1-3 hours.

The atmosphere inside the container can also be an atmosphere with a lowoxygen content (less than 20% atomic) In particular, the percolation canbe effected by infiltration of the porous preform of activated boronpowder, immersed in molten magnesium, maintained under a pressure ofinert gas.

The percolation can also be effected in a sealed container, at atemperature which is sufficiently high and a gas pressure which is suchas to allow the liquid magnesium to wet the activated boron powder,constantly in the absence of oxygen or with a minimum oxygen content.

The following detailed description of the method according to thepresent invention provides that the preform of activated crystallineboron powder, the necessary quantity of metallic Mg, be inserted insidethe container—a container which, for the sake of simplicity, can be madeof steel suitably protected with the sheath described above, preventingit from being attacked by the magnesium and boron at hightemperatures—remaining trapped in an atmosphere of inert gas or with alow oxygen content, at such a pressure as to guarantee the presence ofmagnesium in liquid phase at the reaction temperatures. The metallic Mg,present in such a quantity as to have an atomic ratio Mg/B greater than0.5, must be arranged so as to allow, one the high temperatures, i.e.over 650° C., have been reached, the percolation of the liquid magnesiumthrough the boron preform.

The crystalline boron used in the present invention has a prevalentcrystallinity of the rhombohedral type characterized by the presence ofat least two distinct phases for different unit cell parameters: it mustbe previously mechanically activated, so as not to modify thecrystallinity itself and obtain a particle-size which is such as to bemore rapidly and more effectively permeated by the liquid magnesium. Oneway of activating the boron is by grinding, for example in a press, thecrystalline flakes having dimensions of a few millimetres by high loadcompression crushing under “almost static” conditions, said grindingbeing different from that effected in a rotating ball mill. This lattertype of grinding, in fact, not only produces powders with a much finerparticle-size (lower than 20 micrometres), but also induces undesiredvariations in the crystallinity of the starting crystalline boron, saidvariations being detected by means of X-ray diffraction from powders, asthe disappearance of the splitting of the diffraction lines, leaving theknown rhombohedral crystalline boron phase alone (described in databaseJCPDS, card #11-618): this phenomenon is associated with thedisappearance of a larger unit cell phase, present in the startingcrystalline B flakes, whose presence can be considered as beingfavourable for the permeation of the magnesium.

The preform of activated crystalline boron powders can be prepared withthe usual powder compacting techniques and must have an appropriateapparent density. The preform can alternatively be produced in thecontainer itself by pouring the activated crystalline boron powderdirectly inside and compacting it until the desired apparent density isreached.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As specified above, the preform of activated crystalline boron powdercan contain up to 20% atomic of magnesium and can consist of activatedcrystalline boron powders, surface-coated by metallic Mg.

It has been surprisingly found that the use of preforms suitablyprepared as described above, closed inside a sealed container containingappropriate contents of inert gas or with a low oxygen content andmaintaining the reagents at temperatures higher than 700° C. for atleast 30 minutes, allows the reactive transformation of B and Mg formingMgB₂ and minority metallic Mg in the whole volume already occupied bythe preform. The products are homogeneously distributed, also inside theend-products, with the occasional presence of empty zones having averagedimensions of less than 20 micrometres. Neither the presence of metallicmagnesium nor the presence of empty zones has a significant influence onthe extraordinary superconductor characteristics of the end-products.

By using as reagent, instead of pure liquid Mg, a mixture of this withone or more lower-melting metals, such as for example Ga, Sn, In and Zn,or an equivalent alloy, the latter present in the desired quantity up tothe percentage corresponding to the eutectic point of the alloy, it isequally possible to produce highly densified end-products of MgB₂ havinganalogous superconductor properties to those obtained using puremetallic Mg.

The presence of minority phases, foreign to the crystalline lattice ofMgB₂ and due to the metals used in the alloy, has proved not to beobstacle for the superconductivity. The use of these alloys, havingmelting points lower than that of pure magnesium, by reducing theviscosity of the liquid metal at the typical reaction temperatures,allows the reaction to take place in more rapid times and/or at lowertemperatures and is therefore a useful method for reducing the processcosts.

The main advantage of the method according to the present invention, aspreviously observed, consists in that it allows the production, in asimple and economic way, of superconductor solid end-products of MgB₂,densified up to values close to the theoretical value, with improvedcharacteristics with respect to the products obtained with the knownmethods in the state of the art. From an applicative point of view, theuse of MgB₂, densified up to values close to the theoretical value, thusobtained, allows the current which can be conveyed into thesuperconductor solid end-products, to be increased and also improvestheir mechanical characteristics.

The following examples are provided for a better understanding of thepresent invention.

EXAMPLE 1

20 g of activated crystalline boron powder are prepared starting fromflakes of crystalline boron having dimensions of a few millimetres(purity 99.4%, of commercial origin: grade K2 of H. C. STARK, Goslar(D)), grinding the flakes by applied high load crushing, i.e. by placingthem between two metallic plates situated between the pistons of apress, to which loads of up to 50 tons are repeatedly applied, under“almost static” conditions. The powders thus ground are sieved with a100 micrometre mesh sieve. The X-ray diffraction spectrum of the powdersthus sieved, still has splitting, on the part of the higher interplanardistances, of the diffraction peaks typical of the crystalline boronphase (rhombohedral cell described in the file JCPDS, card#11-618corresponding to pseudohexagonal cell sides a_(o)=1.095 nm, c₀=2.384nm). The supplementary diffraction peaks, present in the activatedpowder, have an intensity comparable with those of the rhombohedralphase and can be interpreted as belonging to a phase having a cellsimilar to a rhombohedral cell, corresponding to pseudohexagonal cellsides a_(o)=1.102 nm, c_(o)=2.400 nm, with a consequent average volumeexpansion of 1.8%, with respect to the regular rhombohedral crystallineboron phase. As an example, the splitting of the first five reflexes canbe observed in the X-ray diffraction diagram of powders represented inFIG. 1 (thick line) which also indicates for comparative purposes (thinline) the corresponding reflexes of a boron powder obtained from thesame starting flakes, but ground with a conventional method, i.e. with arotating ball mill.

EXAMPLE 2

A cylindrical steel container, schematically illustrated in FIG. 2, islined with a sheet of Nb having a thickness of 100 micrometres (FIG. 2wherein 1 indicates the steel container and 2 the protective sheaths).The sheet is wrapped twice around the internal wall and two disks of Nbhaving the same thickness are placed on the bottom and below the plug ofthe steel cylinder. Two magnesium cylinders, having a total weight of15.2 g, a purity of 99% and a diameter which is such as to allow them tobe accurately inserted inside the Nb sheath, are subsequently insertedinto the container thus lined; 10.7 g of the activated crystalline boronpowder of Example 1 are placed between the above two Mg cylinders andcompacted by gravity, with an apparent density equal to 52% of thetheoretical density of rhombohedral crystalline B.

The weights of the reagents are such as to obtain an atomic ratio Mg/Bequal to 0.63.

The steel container is placed in a stream of Argon and then sealed bywelding the plug to the electrode. It is then placed in a quartztubewhere it is heated, in a stream of Argon, to a temperature of 950° C.,for 3 hours. The gas entrapped in the steel container: generates apressure of about 4 atmospheres at 950° C., sufficient for ensuring thestability of the liquid Mg phase in equilibrium with MgB₂ (see thearticle of Zi-Kui Liu et al. Preprint in Condensed-Matter Publ. Nr.0103335, March 2001).

After cooling, the metallic container is opened and a homogeneouslydensified cylinder, having a density of 2.4 g/cm³, a diameter of about17 mm and a height of about 30 mm, is removed from the central part.Analysis by means of X-ray diffraction from powders, represented in FIG.3, verifies that said densified cylinder mainly consists of MgB₂, withthe presence of a minority phase of metallic Mg and other minoritypeaks, non-identifiable but in any case not attributable to MgO.

A part of the MgB₂ cylinder thus obtained is then removed to control itscritical temperature by measuring the magnetic susceptibility inalternating current, represented in FIG. 4, verifying that thesuperconductive transition has an incipient Tc of 39 K and thebroadening of the curve, in the inflection point, is ΔT=0.5 K.

A rectangular bar with a section equal to 6.2 mm² and a length equal to28 mm, is then cut from the MgB₂ cylinder, and resistive measurements ofthe critical current are effected in the presence of high magneticfields at a temperature of 4.2 K.

With the criterion of the critical current measurement at the electricfield corresponding to 100 microvolts/m (European regulation EN61788-1:1998, the values of Table 1 were obtained:

TABLE 1 Critical stream density Magnetic field (Tesla) (A/mm²) 9 29.0 1012.0 11 4.5 12 2.2

EXAMPLE 3 Comparative

Following the same procedure described in Example 2, an analogouscontainer is prepared, using the same quantity of Mg and 11.58 g ofcrystalline boron powder, of the same origin as that of Example 1, butnot activated according to the procedure described in Example 1. Theatomic ratio between the Mg/B reagents is therefore equal to 0.58. Thecrystalline boron powder was ground conventionally in a rotating ballmill and sieved with a sieve having a mesh of 100 micrometres. Thepowder, which is much finer, is compacted to an apparent density valueequal to 57% of the theoretical density of rhombohedral crystallineboron.

After thermal treatment analogous to that of Example 2, the resultingproduct is removed from the container, consisting of two densifiedcylinders of MgB₂, having a diameter of 17 mm and a height of about 8mm, and partially reacted boron powder, situated between, the twodensified cylinders.

EXAMPLE 4

The procedure described in Example 2 is followed, both for thepreparation of the container and for the nature and method of use of theactivated crystalline boron powder. In addition to two cylinders ofmetallic Mg, two disks of metallic Zn (purity 99%) are also used, inaccordance with the following overall quantities: Mg=5.91 g, Zn=4.64 g,B=5.10 g. The following atomic ratios are therefore used:(Zn+Mg)/B=0.67; Mg/B=0.52; Zn/Mg=0.29.

The activated crystalline boron powder was compacted in the container toan apparent density of 54% of the theoretical value of rhombohedralcrystalline boron.

After thermal treatment carried out at 850° C. for 2 hours, ahomogeneously densified cylinder is removed from the container, having adiameter of 14 mm and a height of 22 mm and a density=2.57 g/cm³, which,upon X-ray diffraction analysis, proves to mainly consist of MgB₂, withminority phases containing Zn.

A part of the cylinder of MgB₂ thus obtained is then removed to controlits critical temperature by measuring the magnetic susceptibility inalternating current, FIG. 5, verifying that the superconductivetransition has an incipient Tc of 38.4 K and the broadening of thecurve, in the inflection point, is ΔT=1.0 K.

1. A method for the preparation of superconducting massive bodies ofMgB₂, having a density close to the theoretical value, which comprisesthe following steps: a) mechanically activating crystalline boron toform activated powders; b) forming a porous preform of activatedcrystalline boron powders; c) assembling the porous boron preform andmassive precursors of metallic magnesium in a container and sealing saidcontainer in an atmosphere of inert gas or an atmosphere with a lowoxygen content, wherein said sealed container is capable ofcounterbalancing the internal pressure in said container which isgenerated during thermal treatment and is also capable of preventing thecontents of said container from contamination with atmospheric oxygenduring thermal treatment; d) thermally treating the boron and magnesiumas assembled above, at a temperature higher than 700° C. for a timegreater than 30 minutes, with the consequent percolation of themagnesium, in liquid phase, through the activated crystalline boronpowders.
 2. The method according to claim 1, characterized in that themechanical activation passage a) of crystalline boron consists ingrinding flakes of crystalline boron by repeated crushing effected byhigh load compression.
 3. The method according to claim 1, characterizedin that the activated crystalline boron powders have an averagevolumetric particle diameter ranging from 30 to 70 micrometers and havea type of crystallinity equal to that of the starting crystalline boronflakes.
 4. The method according to claim 1, characterized in that thepreform of activated crystalline boron powders is prepared with theusual powder compacting techniques.
 5. The method according to claim 1.characterized in that the preform of activated crystalline boron powdersis prepared in the container itself by directly pouring the activatedcrystalline boron powder inside and compacting it.
 6. The methodaccording to claim 1, characterized in that the preform of activatedcrystalline boron powders has an apparent density higher than 50% of thetheoretical density of the crystalline boron (2.35 g/cm.sup.3).
 7. Themethod according to claim
 1. characterized in that the preform ofactivated crystalline boron powders has a purity higher than or equal to99.4%.
 8. The method according to claim 1, characterized in that thepreform of activated crystalline boron powders has a shape similar tothat of the end-product.
 9. The method according to claim 1,characterized in that the preform of activated crystalline boron powderscontains up to 20% atomic of magnesium in the form of magnesium powderhaving a particle-size lower than that of boron.
 10. The methodaccording to claim 1, characterized in that the preform of activatedcrystalline boron powders consists of activated crystalline boronpowders, surface coated by metallic magnesium.
 11. The method accordingto claim 1, characterized in that the combining step c) of the porousboron preform and massive precursors of metallic magnesium in acontainer is effected with massive precursors of metallic magnesiumhaving a purity higher than 99%.
 12. The method according to claim 1,characterized in that in step c) there is an excess of Mg which is suchthat the atomic ratio Mg/B is greater than 0.5.
 13. The method accordingto claim 1, characterized in that the atomic ratio Mg/B is higher thanor equal to 0.55.
 14. The method according to claim 1, characterized inthat the container used in step c) consists of a material which cannotbe attacked by the boron and magnesium at temperatures up to 1000° C.15. The method according to claim 14, characterized in that the materialis Nb, Ta, MgO, BN.
 16. The method according to claim 1, characterizedin that the container used in step c) consists of any material resistantto high temperatures, internally lined by a sheath of a material whichcannot be attacked by the boron and magnesium at temperatures up to1000° C.
 17. The method according to claim 1, characterized in that stepd) comprises thermal treatment at temperatures ranging from 800° C. to1000° C., for 1 to 3 hours.
 18. The method according to claim 16, wherethe container material used in step c) is steel.
 19. The methodaccording to claim 16, where the internally lined sheath materials areselected from Nb, Ta, and MgO.
 20. The method according to claim 1,characterized in that in step c) the massive precursors of metallic Mgconsist of massive bodies of magnesium and one or more lower-meltingmetals or equivalent alloys.
 21. The method according to claim 20,characterized in that the lower-melting metals arc present in such aquantity as to reach as far as possible the percentage corresponding tothe eutectic point of the equivalent alloy.
 22. A method for thepreparation of superconducting massive bodies of MgB₂, having a densityclose to the theoretical value, which comprises the following steps: a)mechanically activating of crystalline boron with the formation ofactivated powders; b) forming a porous preform of activated powders ofcrystalline boron; c) assembling, the porous boron preform and massiveprecursors of metallic magnesium consisting of massive bodies ofmagnesium and one or more lower-melting metals or equivalent alloys thatmelt at a temperature lower than the melting temperature of magnesium ina container and sealing thereof in an atmosphere of inert gas or with alow oxygen content; d) thermally treating the boron and magnesium asassembled above, at a temperature higher than 700° C. for a time greaterthan 30 minutes, with the consequent percolation of the magnesium, inliquid phase, through the activated crystalline boron powders the methodaccording to claim 19, characterized in that wherein the atomic ratio ofthe lower-melting metal+metallic magnesium/boron is greater than 0.55and contemporaneously the atomic ratio of magnesium/boron is greaterthan 0.5.
 23. The method according to claim 20, characterized in thatthe lower-melting metals are selected from Ga, Sn, In and Zn.